Cryopumps are the pump of choice when large pump speeds are needed for ultra-high vacuum applications, particularly in areas where magnetic fields would affect the performance of turbo or ion pumps. Cryopumps are ideal for those applications needing large pump speed. Cryopumps are used to pump vacuum systems to pressures typically no lower than 10−10 Torr, with limitations due to outgassing from the pump materials and water vapor that is not completely eliminated from the cryopump itself. The pump works by removing molecules from the vacuum system by adsorption on the cryosorption material in the cryopump, which is a large surface area material such as charcoal or xeolite which is attached to the cryosorption surface using adhesives.
An extreme high vacuum (XHV) bakable cryopump would be very useful. XHV conditions are becoming prevalent in molecular beam epitaxial machines, electron sources for accelerators and light sources, reactive surface science experiments, nuclear and high energy physics detectors and scattering chambers, and atomic applications such as atomic traps. Existing XHV pumps have limitations such as particulate matter and inability to pump noble gasses (non-evaporable getter pumps) and ion backstreaming (ion pumps). Extreme high vacuum as defined herein is pressure less than 10−12 Torr or 10−10 Pa.
Cryopumps typically have two parts: a means for cooling and a cryosorption surface for adsorbing molecules. The cooling can be done in many ways, including closed cycle helium refrigerator systems, liquid helium baths or liquid helium flow. The cryosorption surface is within the vacuum system, in thermal contact to the cooling mechanism, and provides surface area for gas molecules to adsorb and leave the gas phase, reducing the number of gas phase molecules in the system and correspondingly lowering the pressure of the system.
Unfortunately, use of a conventional materials for cryosorption material imparts a serious limitation on the cryopump as it limits the baking temperature of the cryopump. Due to the melting temperature of the adhesive used to adhere the cryosorption material to the cold surface, the cryopump cannot be baked above a temperature typically near 50° C., and thus the base pressure achieved by the cryopump is limited by the water vapor and adhesive vapor pressure. One specialty cryopump avoids the problem of heating the cryosorber adhesive during bake by adding a liquid nitrogen chill circuit to cool the cryosorber panels during the bake of the system, and aims to be able to reach a pressure below 10−12 Torr. However, this pump is difficult to use and is susceptible to cryogenic failure during the bake, which could cause catastrophic loss of cryosorber material in the pump and furthermore does not eliminate the water contained in the conventional cryosorber material as it limits the bake temperature to below 50° C.
What is needed therefore is a cryosorption material that does not place any limitations on the baking temperature of the cryopump. This would facilitate using cryopumps to reach XHV pressures.